We run all of our fans at a flat 80%. Seems to be a good compromise between good airflow, and not killing the fans.
Tried using 80% fan speed and about 6 out of 20 fan died after 3 months.... is it because of the sapphire brand I am using?
Sapphire fans are complete shit. All my cards are Sapphire and all of the fans have died in under a year using 100%. Now I use 80% so we shall see.
The expected lifetime of any fan, regardless of the brand, is likely measured in months if it is continuously running at maximum speed.
I would agree with that if we're talking about stock fans. But if you buy a quality fan, and properly maintain it, you're talking many years of good use so long as you don't end up with a lemon. The amount of mass these computer fans have to throw around is relatively low which means less stress on those moving parts versus a larger fan. And there are larger fans that have been around since the the 1920's. They don't make em' like they used to.